Waves of up-and-down winds that span great ranges in air
pressure may explain the surprisingly clear, dry areas near
Jupiter's equator, new research based on data from NASA's Galileo
entry probe indicates.

Scientists have been trying to understand the stability of
these clear "hot spots" ever since the probe plunged into one of
them nearly five years ago.

"If you could ride in a balloon coming into one of the hot
spots, you would experience a vertical drop of 100 kilometers
(about 62 miles) -- more than 10 times the height of Mount
Everest," explained Dr. Andrew Ingersoll, a Galileo science team
member at the California Institute of Technology in Pasadena.

An explanation of how these deep holes in Jupiter's clouds
could persist is reported in today's edition of the journal
Science by Dr. Adam Showman, of NASA's Ames Research Center,
Moffett Field, Calif., and Dr. Timothy Dowling, director of the
University of Louisville's Comparative Planetology Laboratory in
Kentucky.

"This helps answer one of the big puzzles we ended up with
after the probe entry," said Dr. Torrence Johnson, Galileo
project scientist at NASA's Jet Propulsion Laboratory in
Pasadena.

Showman and Dowling propose that air moving west to east
just north of Jupiter's equator is also moving dramatically up
and down every few days. Water and ammonia vapors condense into
clouds in Jupiter's white equatorial plumes as the vapors rise.
Then the wrung-out air drops, forming the clear patches. After
crossing those hot spots, the air rises again and returns to its
normal cloudy state.

The researchers developed a computer simulation that
recreates known traits of the hot spots and plumes when the
simulation starts with a large-scale pressure difference. Dowling
said smaller pressure differences do not produce stable patterns.

"There are no wimpy hot spots, only strong ones," he
quipped.

During the Galileo probe's hour-long descent on Dec. 7,
1995, it returned the only direct measurements ever made from
within Jupiter's atmosphere. Scientists quickly realized the
entry point was a special place. On a planet mostly wrapped in
high clouds, the probe hit the southern rim of a clear spot where
infrared radiant energy from the planet's interior shines
through.

The computer simulation reveals that the probe's entry site
is probably even more unusual than previously thought. Both the
probe and the computer model show that the head winds on the
southern rim of a hot spot get stronger and stronger with depth
into the planet. But in the model, this trend is reversed on the
northern rim. "These results underscore the importance of future
multi-probe missions to Jupiter," said Dowling.

The hot spots were known previously, but their depth was a
surprise, noted Ingersoll. A better name for them might be bright
spots, since the temperature at their visible depth is only about
32 degrees Fahrenheit (zero Celsius), though that is relatively
balmy compared to the neighborhood of minus 200 degrees
Fahrenheit (minus 130 degrees Celsius) at surrounding cloud tops.

Each hot spot is about the size of North America and lasts
for months. The hot spots alternate with larger cloudy plumes in
a band near Jupiter's equator. In some ways, the dry areas where
wrung-out air masses are descending resemble subtropical deserts
on earth, Ingersoll said. But, unlike Earth, Jupiter has no firm
surface to stop the air's fall.

All the hot spots combined make up less than one percent of
Jupiter's global area, but understanding how they remain stable
is important for understanding the whole planet's atmospheric
dynamics, Dowling said.

Also, the hot spots have "mathematical cousins" in some
equatorial movements in Earth's oceans and atmosphere, he said.
"How distantly or closely related they are is a question we are
just beginning to study."

The Galileo mission includes an orbiter that has been
studying Jupiter and its moons since it finished relaying
information from the atmospheric probe nearly five years ago. The
mission is managed by JPL for NASA's Office of Space Science,
Washington, D.C. JPL is a division of the California Institute of
Technology.